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FOURTH ANNUAL
ASAP '96 WORKSHOP

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Cancellation of Terrain
Scattered Jamming In
Airborne Radars

Larry Brennan
Larry Brennan and Associates
9196 Neumann Drive
Elberta, AL 36530
tel: (334) 986-6241
fax: (334) 986-6241
email: lbrennan@gulftel.com

Abstract Multipath jamming, scattered from terrain into a radar main beam, cannot be canceled by angular nulling or angle-Doppler STAP techniques. However, scattered jamming cancellation has been demonstrated using an adaptive filter and consecutive samples (range cells) from an auxiliary beam pointed at the jammer. Alternatively, the necessary auxiliary inputs can be obtained from multiple beams of scattered jamming. In both cases, the adaptive filters approximate the terrain scattering process, providing a signal for subtraction from the main beam. In airborne radars, the scattering process is continually changing, requiring rapid weight updates. Radar clutter must also be canceled in airborne radars, requiring that multiple PRIs of data be combined coherently in some form of Doppler filtering. Clutter-free radar outputs are needed in computing the adaptive weights for scatter cancellation. Methods of satisfying these two requirements (rapid weight updates and long coherent processing intervals) simultaneously are discussed. One option is a two- or three-pulse MTI canceler in the main beam, followed by scatter cancellation. If necessary, the scatter canceler weights can be updated each PRI or several times per PRI. A second option is DFT Doppler processing over many PRIs for clutter removal, followed by scattered jamming cancellation. The clutter-free filter outputs are then used to obtain rotating scatter canceler weights which compensate for the changing bistatic path lengths. The Mountaintop data obtained with an airborne jammer at White Sands Missile Range was used to test these algorithms. Results of these tests are presented and discussed.

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